The invention relates to a semiconductor laser comprising a grating and a waveguide layer applied to the grating by LPE (liquid phase epitaxy) as well as a plurality of layers disposed above the waveguide layer. Semiconductor lasers are known, particularly DFB (distributed feedback) lasers which have a flat first order grating that is preferably wet chemically etched. By way of MOVPE (metalorganic vapor phase epitaxy), a sequence of layers is applied to the grating, with a waveguide layer being grown directly on the grating. The application of the layers by MOVPE makes it possible to apply very thin active layers onto the waveguide layer which, particularly in connection with 1.55 .mu.m lasers, results in a high quantum yield and high output power. However, the application of the waveguide layer to the grating by means of MOVPE is possible, without adversely affecting the grating, only if the depth of the grating is not particularly distinct. The outcome is then, however, that the coupling coefficient of the semiconductor laser is not the best.
It is known to improve the coupling coefficient by means of deep second order gratings, in which case the shape of the grating is preferably given a special configuration. That means, the slopes of the individual raised portions of the grating are inclined at a specific angle. Moreover, the top-to-bottom ratio and the depth of the grating are selected in such a manner that, as a whole, an optimum coupling coefficient results. This improves, in particular, feedback sensitivity of the semiconductor laser.
However, it has been found that the waveguide layer cannot be applied to such gratings by means of MOVPE. The temperatures required for this process have an adverse influence on the grating, that is, the individual ribs of the grating become rounded so that the values selected for the slope angle and the grating depth in order to obtain the optimum coupling coefficient no longer exist once the waveguide layer has been applied.
Moreover, it is not possible to apply very thin active layers to the waveguide layer by LPE and therefore the quantum yield and the optical output power of a laser produced by LPE are insufficient. An added drawback of LPE is that a dissolution preventing layer must additionally be applied to the active layer before further layers are grown. This increases the effective thickness of the active layer.